As transmission techniques for increasing communications speed of wireless LANs, MIMO/SDM (Multiple Input Multiple Output/Space Division Multiplexing), MIMO/SM (Multiple Input Multiple Output/Spatial Multiplexing), and other MIMO communications systems are considered promising. In simultaneous communication, by installing multiple transmitting antennas and receiving antennas, assigning different channels in the same frequency band to different transmitting antennas, and transmitting different sequences of signals to the different channels simultaneously, it is possible to increase transmission speed without expanding the frequency band. Thus, even if the frequency band is not expanded, it is possible to increase sequences of transmission signals with increases in the number of transmitting antennas, and thereby improve the usability of frequencies and increase the wireless transmission speed. To this end, Japanese Patent Application No. 2001-119238 describes an antenna device comprising a first planar inverted F antenna and a second planar inverted F antenna installed symmetrically about a printed circuit board.
Thus, to implement a MIMO communications system, one communications device must have multiple broadband antennas, and when installing multiple antennas, as recognized herein it is necessary to provide sufficient space among the antennas to avoid interference among the antennas. The present invention understands that in MIMO communications systems, when n antennas constitute independent frequency channels, if data transfer speed per channel is A (bps), the data transfer speed T (bps) of all the antennas is nA. However, as recognized herein if there is interference among the antennas, the data transfer speed T is smaller than nA.
Recently, mobile information terminal devices have come into wide use, requiring high transmission speed even from mobile personal computers, PDAs, cell phones, or the like, but as recognized by the present invention, on small information terminal devices, it is difficult to provide enough space between antennas to reduce interference among them. Furthermore, the present invention recognizes that the size of the antennas used for small information terminals should be minimized as much as possible. Additionally, as understood by the present invention, to overcome spatial constraints and to mount a MIMO-compatible antenna on a small information terminal, it is convenient that the antenna be an integral-type multi-element antenna with multiple radiating elements formed in a single package. With these critical observations in mind, the invention herein is provided.